Pore water distributions of dissolved copper and copper-complexing ligandsin estuarine and coastal marine sediments

The distributions and seasonal variability of total dissolved Cu (TDCu) and Cu-complexing ligands in sediment pore waters have been investigated at tw o contrasting sites in the Chesapeake Bay. Two ligand classes, which differ on the basis of the conditional stability constants (K'(cond)) of their Cu complexes, were detected at all depths at both sites. At the sulfidic, mud dy, mid-Bay Sta. M, concentrations and values of log K'(cond) ranged from 3 90-12,500 nM and greater than or equal to 7.2->8.9, respectively, for the s tronger ligand class (L-1S) and 75-6,420 nM and 6.2-7.9 for the weaker liga nd class (L-2S). At the bioturbated, sandy Sta. S in the lower Bay, respect ive concentrations and values of log K'(cond) ranged from 135-807 nM and gr eater than or equal to 7.6-greater than or equal to 10.2 for L-1S and 40-1, 410 nM and 6.6-9.2 for L-2S. For comparison, one pore water profile from a slope station off of the Chesapeake Bay also showed the presence of two lig and classes, with respective concentrations and values of log K'(cond) of 1 40-270 nM and 8->11 for L-1S and 30-180 nM and 7-10 for L-2S. These ligands are in large excess relative to ambient TDCu concentrations (<0.1-24.3 nM) , thereby maintaining very low inorganic Cu concentrations (typically <0.1 to <100 pM) and a high degree of organic complexation (87.2->99.9%) of Cu i n Bay and slope sediment pore waters. Thus, virtually all TDCu fluxing from these sediments is complexed during sediment-water exchange. A relatively small fraction of the TDCu is exchanged as inorganic species, which are wid ely regarded as the most bioavailable form of Cu. Higher ligand concentrati ons at Sta. M suggest that sulfide or organic ligands containing reduced S contribute to the pool of complexing ligands; however, the exact nature and sources of the ligands in Bay pore waters are not known. The progressive i ncrease in conditional stability constants of the CuL2S complexes from the mid-Bay to the slope sediments may reflect differences in biological or che mical processes at each site, as well as differences in the type of Cu-comp lexing organic matter. Total ligand concentrations (L-1S + L-2S) are 15 to >100 times higher in the upper intervals of the pore waters relative to lig and concentrations in the bottom waters of the Chesapeake Bay (30-60 nM), c onsistent with previous observations of fluxes of these ligands from the se diments to overlying waters. These results suggest that sediments are poten tially significant sources of Cu-complexing ligands to the overlying waters of the Chesapeake Bay, and perhaps, other shallow water estuarine and coas tal environments. Copper-complexing ligands released from sediment pore wat ers may play an important role in influencing Cu speciation in overlying wa ters. Copyright (C) 2000 Elsevier Science Ltd.